The use of newly discovered anisotropic etching processes has led to major advances in the manufacture of microelectronic devices. The ability to form openings in dielectric and metal layers with very precise dimensions, and, where needed, with optimally small dimensions enables device packing densities unobtainable without these processes. Moreover, anisotropic techniques have increased processing reliability. This leads to higher device yields and attendant lower device costs. Nearly all leading edge VLSI silicon devices will be made using these techniques.
The physical mechanism by which straight sidewalls result from reactive ion etching (RIE) processes has been debated for some time. A widely accepted explanation is the so-called recombinant theory advanced by Mogab and co-workers at AT&T-Bell Laboratories. Their explanation involves a choice of chemical species for the gaseous plasma environment that includes a recombinant species. The recombinant species chemically reacts with the active etchant species after the first energetic collision. Energetic collisions are controlled by the electric field, and they are consequently directional. They occur primarily on sites that are normal to the beam direction. Secondary collisions, that would occur on the sidewalls and produce sidewall etching, are effectively prevented by the presence of the recombinant. Another theory is described in U.S. Pat. No. 4,436,584, issued Mar. 13, 1984 to Bernacki et al, and attributes the absence of sidewall etching in anisotropic techniques to the formation of a polymer film on the sidewalls. Due to some property of the beam associated with its directionality, the polymer does not form on surfaces normal to the beam.
I have discovered experimentally that the etching behavior of so-called anisotropic systems is strongly influenced by the angle of the sidewall of the layer used as a mask. My findings are similar to those reported in the aforementioned patent of Bernacki et al. In one of my experiments an asymmetric photoresist mask was used. One side of the mask had an essentially vertical sidewall, the other side had a sloped sidewall. The layer being etched, which in this experiment was a silicon dioxide layer, developed a vertical sidewall under the mask edge that was essentially vertical, but the oxide beneath the sloped mask edge showed a decided undercut. This provides clear evidence that the degree of anisotropy depends on the slope of the wall of the masking layer and not primarily on any of the variety of variables in the etch process itself. It then became apparent that steep sidewalls on the photoresist pattern would be desirable for anisotropic etching processes. However, the sidewalls on typical photoresist patterns tend to slope.